1. Field of the Invention
The present invention relates to workpiece holding devices for holding a tool or workpiece in a workpiece processing assembly.
2. Description of Related Art
Workpiece holding devices, such as collet and jaw chucks, are used to clamp or grip workpieces or tools in workpiece processing assemblies such as turning machines. The workpiece holding devices include a number of circumferentially-spaced gripping segments that are movable between expanded and contracted positions to grip or release a workpiece or tool. A closer is operatively connected to the workpiece holding device to apply a force for moving the segments of the workpiece holding device between the expanded and contracted positions. Typically, a draw bar is rigidly coupled between the workpiece holding device and the closer such that actuation of the closer applies a force to the workpiece holding device via the draw bar.
The closer is often times in the form of a hydraulic closer. Hydraulic closers are advantageous in that they can exert large forces (with respect to pneumatic closers, for example), which in turn will grip the workpiece or tool very securely for a processing operation. However, when a delicate workpiece, such as a thin walled ring, is to be processed, the large forces exerted by the hydraulic closer easily distort the delicate workpiece. Typically, the operating force in hydraulic closers cannot be lowered enough to eliminate distorting the workpiece. Moreover, lowering the operating force to low levels causes the hydraulic closer to move extremely slow, thereby dramatically slowing down the cycle time for gripping the workpiece.
The present invention provides an improved workpiece holding device that is structured to limit the force that may be applied by a closer to the workpiece holding device so that thin walled or delicate workpieces may be gripped which may be otherwise subject to distortion.